(more pics below)
So the Cruze Eco (which I don't have) has a lower grill block that's automatic; thermostatically controlled shutters, blah blah blah wish I had it.
Here on Ecomodder we typically believe upper grill blocks are better; pushing air up over the smoother top of the car rather than the rough underside. Our cruze had the awful recall done on it; the dealer took a sawzall to the bellypan and removed a 2x3 foot section of it.
Anywho, the photo above shows where I started. The regular Cruze has a lower grill (not shown) and a middle grill which is partially blocked off on the sides, and a tiny upper grill that is totally fake. Behind the fake upper is the air intake (end of the silencer) that feeds around to the airbox, then the turbo, intercooler/ducting, and intake manifold.
I initially wanted to block the middle grill from the outside with lexan but the wife wants all the ecomods to be invisible, so...
I cut a cardboard template based on the outside shape of the hole, then removed the front belly pan. It took eight T-20 Torx screws and four plastic retainers that were removed with a flathead and needle nose pliers. I tried the cardboard template and modified it to fit better and used the template to cut a sheet of coroplast (corrugated plastic campaign sign) to the shape. I painted it with Krylon fusion plastic paint and slid / wedged it in. I used a flathead screwdriver to poke some holes in the coroplast (CAREFUL to not push another couple inches past and into the AC condenser). I was planning on using zip ties (photo) to hold it in, but my zip ties were brittle and useless. I grabbed some black 14 gauge wire I had sitting around (solid copper core like the stuff in home wiring) and pushed "U" shaped pieces through from the front and twisted in the back. You can see one in the last picture, about four holes to the left of the Chevy Bowtie. I have a total of four in there and it seems to be holding great. Not too experimental, we've only had the car for 3 weeks and I also fixed a bad spark plug gap today. I know theoretically and due to some other folks' wonderful research that it will work.

Thanks Daox.
I just ordered a wolverine stick-on oil pan heater (car has no freeze plugs and the oil pan is aluminum, so I can't use a magnetic heater!)
Also going to replace the belly pan where GM cut it out, this time with sheet metal I have (old real estate signs)

How much of a gain have you seen in fuel economy after installing the grill block? I have a 13' cruze and have been thinking about doing the same thing. Any issues with radiator fan kicking on? Thanks in advance